This invention relates generally to implantable devices, and, more particularly, to implantable medical devices having therapeutic or diagnostic functions and related methods.
Diseased or defective portions of a patient""s vascular system may be treated or replaced to correct or improve the patient""s health. For example, persons requiring periodic hemodialysis to compensate for poor or absent renal function frequently are provided with arteriovenous grafts or shunts that couple a vein to an artery to facilitate coupling the patient to the dialysis unit. Dialysis, in turn, provides significant health benefits to the patient. In other cases, diseased portions of vasculature are replaced with or supplemented via grafts to facilitate blood flow or to reduce risk of rupture of an aneurysm. The grafts may comprise natural materials, e.g., a portion of a blood vessel taken from another area of the patient""s body, or they may comprise artificial materials, such as DACRON(trademark), TEFLON(trademark)or GORE-TEXT(trademark) fabric. In still other cases, angioplasty is used to alleviate stenosis of major blood vessels. In all of these cases, narrowing of the vessel or graft lumen (stenosis or restenosis) is likely, whether from thromboses, deposited material or tissue growth within the treated area (endothelialization).
Following graft implantation, or surgery to relieve stenosis, it is difficult to monitor the condition of the affected area. Grafts and angioplastic surgery often fail after a period of time and for a variety of reasons. Several of the causal mechanisms are amenable to drug treatment. It is highly desirable in at least some of these cases to localize the drug treatment to the site of the graft or surgery. For example, when stenosis due to thrombosis threatens a given area, thrombolytic drugs are capable of providing significant assistance in resolving the thrombosis, but may present problems such as hemorrhaging, if they also act in other portions of the patient""s body.
In other cases, steps may be taken to restore full fluid flow through, e.g., a graft that is becoming restricted, but only if treatment is initiated before the problem proceeds too far to be corrected without graft replacement. Since it is generally not possible to determine the condition of blood flow through a graft or vessel without invasive surgery to inspect it (or an angiogram), the procedure adopted with some grafts (e.g., access grafts for hemodialysis) is to replace the graft annually. Clearly, it would be preferable to be able to monitor the condition of a graft without resorting to invasive surgical procedures and without prescribing medication that may not be necessary, so that the useful life of the graft may be extended, problems associated graft failure avoided and so that medications are only prescribed when required by the known condition of the graft.
The best indicators of the condition of a graft are the velocity and volume of blood flowing through it. Fluid pressure at the distal and proximal ends of a graft (relative to the direction of blood flow) are a further indication of a graft""s condition. As the lumen through a graft gradually becomes occluded with fatty buildup, other deposits or intima, the pressure differential across the graft increases, the velocity of blood in the lumen decreases and the flow of blood through the lumen decreases. Each of these parameters thus serves as an indication of the condition of the graft and its viability to support necessary blood flow.
Chen et al. propose a light generating system in U.S. Pat. No. 5,445,608. Among its drawbacks are that it requires a photosensitive drug that is activated via light from an implantable probe. A number of different embodiments are disclosed. In one of these, an array of light emitting diodes or solid state laser diodes are mounted on a light bar inside the implantable probe and are energized using either a storage battery power source, an inductively coupled external transformer, or with current provided in leads running through a flexible catheter that extends outside the patient""s body to an external source.
According to principles of the present invention, a device and method are provided to achieve localized drug activation or localized drug delivery on an as-needed basis via an implanted therapeutic transducer that is coupled to an implanted electronic circuit. The implanted electronic circuit effects coupling of signals from a system external to a patient""s body to the implanted electronic circuit via a magnetic coupling. The implanted therapeutic transducer may be activated via control signals transmitted from a control system that is external to the patient""s body. Electrical power may be supplied to the implanted electronic circuit via the magnetic coupling, or, in some cases, via a hard-wired connection.
Optionally, a diagnostic transducer is also implanted that provides diagnostic information describing the condition of blood flow through a vascular graft to which the diagnostic transducer is coupled. This can allow a physician to determine that treatment is needed and then to activate the implanted therapeutic transducer to activate a drug in the vicinity of the vascular graft to which the therapeutic transducer is coupled.
In one embodiment, the therapeutic transducer comprises a cylindrical body including piezoelectric material wherein a first resonance frequency is determined by a thickness of the cylindrical body and a second resonance frequency is determined by a diameter of the cylindrical body. The cylindrical body includes a first electrode coupled to a first end of the cylindrical body and a second electrode coupled to a second end of the cylindrical body. An acoustic isolator may be disposed on the first electrode and on a sidewall of the cylindrical body. The acoustic isolator may comprise a mixture of microballoons and a polymer. The second electrode is coupled to a wall of the vascular graft. The first resonance frequency may be chosen to be related to the second resonance frequency by a factor of two.
In another embodiment, the present invention includes an ultrasonic transducer having a first surface, a sidewall and a second surface. A first electrode is disposed on the first surface. A second electrode is disposed on the second surface. An acoustic isolator having a low relative dielectric constant is disposed on the first surface and the sidewall. An acoustic reflector is maintained in alignment with and facing the second electrode of the ultrasonic transducer. In one embodiment, the acoustic reflector also functions as a permanent magnet or an electromagnet.
Further embodiments include transducers that provide magnetic, electromagnetic, optical, heat, ultrasonic or other kinds of signals for localized activation or delivery of drugs. These embodiments can allow a physician to provide needed therapy without requiring the trauma of surgery in order to maintain viability of a graft. This may also obviate graft replacement that is preventative in nature.
This invention provides an implantable diagnostic device for evaluating blood flow through vasculature on an ongoing basis that is capable of operating over an extended period of time. There is also provided an implantable therapeutic device for providing localized drug delivery or activation on an as-needed basis that is capable of operating over an extended period of time.
This invention provides the advantage that the physician can now locally activate or supply drugs in a variety of situations. For example, in situations where it is determined during surgery that part or all of a growth or tumor cannot be removed for safety or other reasons, the physician will be able to implant a therapeutic device, either on the growth or tumor or on the blood vessels supplying the growth or tumor with blood, to activate drugs directed to, or to selectively supply drugs to, the affected location or tissue. The therapeutic device may be able to be supplied with electrical power from time to time from a location outside the patient""s body.
Yet another advantage that may be realized through practice of the present invention is the treatment of tumors or organs that are downstream of the blood vessel that includes a vascular graft that is coupled to a transducer. The transducer may be remotely activated to facilitate localized drug delivery or to provide other therapeutic benefits.